1. Field of the Invention
The present invention relates to charged particle accelerators, in particular, to charged particle accelerators of RFQ (Radio Frequency Quadrupole) type to be utilized for the analysis of material properties or material composition, surface modification, ion implantation, etc. with the use of beams of high energy charged particles in the fields of process technology of semiconductors, medical care technology, biotechnology, etc.
2. Description of the Prior Art Recently in the manufacturing process of semiconductors, improvement has been made in high integration of circuits on a plane and in accommodating the integrated circuits in multiple layers, and the Rutherford back scattering method (RBS) is used for the analysis of atomic distribution on the IC's as the process research of the above-mentioned IC's.
On the other hand, it is desirable in the manufacture of semiconductor devices, and in particular the surface processing of semiconductor materials to impart special properties such as abrasion proof properties or corrosion resistance properties to the material surfaces. A particle induced X-ray emission method (PIXE) has been developed as a microanalysis method in ppb order, far beyond the conventional analysis precision.
As described above, ion beams (charged particles) are utilized in manufacturing processes or analysis methods. An ion beam of higher energy level is expected to be developed for the improvement of analysis precision of the above-mentioned atomic or molecular distribution in the direction of depth.
In view of the background as mentioned in the above, a linear accelerator which utilizes high (radio) frequency electric field is applied for obtaining a high energy ion beam as mentioned above. In order to improve the transmission efficiency of ions, an accelerator of a radio frequency quadrupole type (hereinafter referred to as RFQ) comprising four vane electrodes (quadrupole electrodes) and a vacuum vessel (a cylinder-shaped container), which works as a resonant cavity having a high Q value, a reciprocal number of the energy loss in a resonant circuit, has been developed.
In FIG. 15, a schematic construction of a conventional RFQ is shown and in FIG. 16, the construction of electrodes is shown.
The electrodes 1, 2, 3 and 4 constituting quadrupole electrodes are disposed in the direction of the center axis of the cylinder-shaped container 5 and the respective surfaces of electrodes 1, 2, 3 and 4 facing each other have uneven corrugated forms. FIGS. 17 (a) and (b) show the sectional views of their relative positions.
In FIG. 17(a), the corrugated forms of facing electrodes are formed in phase and in FIG. 17(b), the corrugated forms of facing electrodes are formed in opposite phase. When a high frequency voltage of specified frequency is applied to the cavity formed inside the container 5 with a loop type coupler 11 as shown in FIG. 18, a high frequency current of the resonant frequency having a mode TE.sub.210 is excited as shown in the figure. In this case, the same electric potential is generated in the facing electrodes and an opposite electric potential is generated in the adjacent electrodes. Because of this, in the vicinity of the axis where four electrodes 1, 2, 3 and 4 are facing each other, basically a quadrupole electric field is generated (not shown in the figure).
In FIG. 18, reference numeral 9 designates the electric field and reference numeral 10 designates the magnetic field.
The explanation about the influence exerted by the above-mentioned corrugated structure in the axis direction of the four electrodes 1, 2, 3 and 4 in the quadrupole electric field as described in the above will be given based on FIGS. 19(a) and 19(b). FIG. 19(a) corresponds to a vertical cross sectional view and FIG. 19(b) corresponds to a horizontal cross sectional view.
For example, in the above-mentioned TE.sub.210 mode when electrodes 1 and 3 are positive, electrodes 2 and 4 are negative, and when the former ones are negative, the latter ones are positive. In addition to such a condition as mentioned in the above, corrugated forms of electrodes 1, 2, 3 and 4 are formed being shifted 180 degrees concerning the horizontal and vertical directions; therefore, for example, when the electrodes 1 and 3 are positive and the electrodes 2 and 4 are negative an electric field in the direction of the center axis is generated on the center axis. The arrows 6, 7 and 8 show the directions of electric fields.
When the polarities of the voltages to be applied to the electrodes 1, 2, 3 and 4 are reversed, the directions of electric fields are also reversed.
For example, when the ions come into the electrode construction along the center axis from the left side in the figure and have a velocity and a phase to be constantly given accelerating electric fields toward the left and the right, the ions are accelerated each time they pass the corrugated formed portions of the electrodes 1, 2, 3 and 4, and their energy is monotonously increased. The ions which at first come into the electrode construction with the phase to be given deceleration are gradually bunched up in the following particles when they pass the next accelerating electric field and after that they are monotonously accelerated.
As described in the above in the case of an RFQ, ions which come in in any phase are finally bunched up and are effectively accelerated.
A strong focusing force is generated in the vertical and horizontal directions by a strong high frequency quadrupole electric field which exists on a plane being perpendicular to the axis, so that ions are accelerated at very high transmissivity.
Actually, the transmission efficiency being close to 100% can not be obtained until electrodes of the optimum design are obtained by changing the period of corrugated forms and the intervals between electrodes little by little in consideration of the increase in ion velocity or of the state of bunching of ions.
In the case of an RFQ as described in the above, the accelerating tube forms a high frequency resonant cavity together with the electrodes 1, 2, 3 and 4, and the resonant frequency (TE.sub.210 mode) is decided by its geometrical dimensions so that it is impossible to largely vary the resonant frequency. The problems in an RFQ which are caused by this structure will be explained in the following.
Generally, in the case of an accelerator utilizing radio frequency waves, ions are accelerated in a state where the travel motion of ions is synchronized with the variation of an accelerating electric field; therefore when the velocity of incident ions is decided for a given kind of ions (e/m), there exists one synchronization condition between an accelerating frequency and the period of the corrugated portions of electrodes; thereby the final accelerating energy obtained with an accelerating tube of a certain length takes an inherent value for a certain kind of ions. In the practical range of tube length and input power, the period of corrugated portions of electrodes is selected to be in the range of several mm to several cm. The above-mentioned RFQ for protons (H.sup.+) is thus set, and has the dimensions of 1.5 m in length and 0.5 m in diameter, and has the resonant frequency of about 100 MHz. If ions, for example, a chemical element As.sup.+, a dopant element for semiconductors, is accelerated in synchronization with the use of an RFQ which can accelerate H.sup.+ up to lMev, the final energy reaches 75 Mev (mass ratio), as an ion energy is expressed by eV=1/2 mv.sup.2 (e: electric charge of an ion, V: accelerating voltage for an ion, m: ion mass and v: ion velocity); it is impossible, of course, to input electric power so as to generate such a high gradient accelerating electric field.
From a different viewpoint, when it is considered to make a 1 Mev accelerator to be used exclusively for As.sup.+ with an RFQ, there are two ways: one is to make the total length 1/75 keeping the frequency as it is and the other is to lower the resonant frequency to 1/75 keeping the length as it is. In the case of the former, the period of the corrugated portions of electrodes must be reduced together with the shortening of the total length which causes a problem in working, and also the intervals between . electrodes (bore diameter) must be reduced to obtain an effective accelerating electric field, which is not suitable for practical use in making the acceptance area for incident ions small. In the case of the latter, to obtain such a low frequency with the same construction as that shown in FIG. 18, the diameter of an accelerating tube must be made 75 times large, which is not practical from a manufacturing standpoint.
In conclusion it is geometrically impossible to make an apparatus as an accelerator for heavy ions for the purpose of industrial utilization with the RFQ of the original type.
In the case of an apparatus for the purpose of obtaining an arbitrary energy level for an arbitrary kind of ions which can be utilized in industry, the accelerating frequency must be variable. In the case of an RFQ, in which the container 5 itself functions as a resonant cavity, the resonant frequency is definitely decided by the geometric form of the container 5, and the setting cannot be arbitrarily changed.
In consideration of such a situation, an accelerator having a function as shown in the following is proposed: an RFQ is provided with an external resonant circuit composed of a variable capacitor and an inductor to be able to accelerate an arbitrary kind of ions to have arbitrary energy level with the supply of high frequency voltage to the electrodes inside the container.
An example of such an accelerator is shown in FIG. 20. The accelerator is indicated in the preliminary manuscript collection for lectures in 36th allied lecture meeting of Applied Physical Society and the related learned societies (second separate volume p 554, Spring, 1989).
As shown in the figure, an external resonant circuit 13 which is provided outside quadrupole electrodes 12 is formed with a cylindrical copper one-turn coil 14 and two variable vacuum capacitors 15 in parallel. High frequency power is led to a coupling capacitor 17 through a coaxial connector 16, and is magnetically coupled to the one-turn coil 14. Both ends of the vacuum variable capacitor 15 are connected to the quadrupole electrodes 12 to contribute to the acceleration of ions.
Besides the above-mentioned apparatus, there is an apparatus having a practical size and able to generate a low frequency voltage for accelerating heavy ions. For example, in the case of a charged particle accelerator shown in FIGS. 21(a) and 21(b), the accelerating tube is excited with a voltage in a TM.sub.010 mode, and from respective end plates 81 and 82 located at both ends of the cavity 80 two beams 83 and 84 are protruded toward the opposing end plate 81 or 82, and these beams are made to be close to each other in the circumference of the center axis to obtain a static capacity C, and respective accelerating electrodes 85 constituting quadrupole electrodes are, as shown in FIG. 20(b), electrically connected to respective beams, 83, 83, 84 and 84, and are fixedly disposed toward the center axis. In the TM.sub.010 mode, lines of magnetic flux 87 are distributed as if they go around the center axis, so that the inductance L can be made large by lengthening the accelerating tube, which makes it possible to lower the resonant frequency.
In the case of an accelerator having an external resonant circuit 13 like the first example of a conventional apparatus shown in FIG. 20, a cable for supplying power to the quadrupole electrodes 12 from the external resonant circuit 13 has stray inductance and stray capacitance which cannot be ignored and also the Q value is degraded by the loss in the cable.
In order to lower a resonant frequency it is necessary to enlarge the diameter of a coil or to increase the capacitance of a capacitor in a resonant circuit; in any way, the geometrical form/size differs much from thin and long RFQ electrodes, and cable wiring for a relatively long distance is needed. When wiring is hung in the air, it is exposed to external disturbances and the apparatus becomes unstable; when wiring is cabled with a coaxial cable or the like, large stray capacitance cannot be avoided.
In order to make the inductance component of an accelerating cavity (container) large, it can be considered to provide an additional electrode of a coiled form inside the cavity or to deform the supporting members for supporting the tip portions of the quadrupole electrodes to coiled forms. It is true that owing to such contrivance a comparatively low resonant frequency can be obtained for the diameter, of its accelerating cavity; in this case however, the path of a surface current in the coil portion becomes long, which decreases the value of Q due to the increase in resistance.
In the case of a second example of a conventional apparatus as shown in FIGS. 21 (a) and 21(b), there are problems as discussed below.
1. A surface current 86 on the surface of the cavity flows to the accelerating electrodes 85 through end plates 81 and 82, but it is difficult to make the electrical connection between the end plates 81 and 82, and the cylindrical cavity complete from the point of views of assembling and maintenance, and the incompleteness often causes lowering of Q or generation of heat at a bad contact point.
2. Each pair of beams among four beams, 83, 83, 84 and 84, are supported with an end plate 81 or 82 in the state of cantilevers, so that the longer is the accelerating tube 80, the harder it becomes to fix the electrodes 85, to be fixed to the beams 83 and 84, with precise relative positions.
3. The surface current 86 induced with a resonant mode flows through the accelerating electrodes 85, and the beams 83 and 84, so that it generates a voltage gradient in the direction of the center axis, which makes it impossible to obtain an ideal RFQ electric field.